(1) Field of the Invention
The present invention relates to magnetoelectric composites and method for preparation thereof which incorporate lanthanum manganese strontium (LaSrMn) oxides or lanthanum manganese calium (LaCaMn) oxides as the magnetostrictive composition in the composite. The composites are in layers, with layers of piezoelectric compositions.
(2) Description of Related Art
In a single-phase material, ME effects require long range ordering of atomic moments and electric dipoles. There are few such material and the effect is often weak (Astrov, D. N., Soviet phys. JETP 13 729 (1961); Rado, G. T., et al., Phys. Rev. Lett. 7, 310 (1961); Foner, S., et al., J. Appl. Phys. 34 1246 (1963); and Kornev, I., et al., Phys. Rev. B 62, 12247 (2000)). For the engineering of materials with new or improved properties Van Suchtelen proposed product-property composites (Van Suchtelen, Philips Res. Rep., 27, 28 (1972)). For example, composites with magnetostrictive (m) and piezoelectric phases (p) are expected to be magnetoelectric because of mechanical stress mediated electromagnetic coupling. Most studies in the past focused exclusively on ferrite-PZT/BaTiO3 composites. Van den Boomgaard synthesized bulk composites of CoFe2O4/NiFe2O4 and BaTiO3 (Van den Boomgaard, J., et al., J. Mater. Sci. 9, 1705 (1974); Van den Boomgaard, J., et al., Ferroelectrics 14 727 (1976); and van den Boomgaard, J., et al., J. Mater. Sci. 13 1538 (1978)). The mixed oxides yielded ME coefficients much smaller than calculated values due to leakage currents through low resistivity ferrites and microcracks that resulted from mismatch of structural parameters and thermal properties. The problem with low resistivity ferrites can be eliminated in a layered structure. Theories predict a very large ME coefficient in a bilayer of p- and m-phases due to enhanced piezoelectricity, but measured values in CoFe2O4-PZT were small (Harshe, G., et al., Int. J. App Electromag. Mater. 4 145 (1993); Avellaneda, M., et al., J. Intell. Mater. Sys. Struc. 5, 501 (1994); and Harshe, G., Magnetoelectric effect in piezoelectric-magnetostrictive composites, PhD thesis. The Pennsylvania State University, College Park, Pa. (1991)). The inventor recently reported giant ME coefficients in bilayers and multilayers of nickel ferrite-PZT (Srinivasan, G., et al., Phys. Rev. B 64, 214408 (2001)), and a record high ME effect was reported very recently in trilayer composites of PZT with Terfenol-D (Ryu, J., et al., Jpn. J. Appl. Phys. 40, 4948 (2001)). The recent theoretical model for a ferrite-PZT bilayer predicts a strong ME effects at microwave frequencies (Bichurin, M. I., et al., Phys. Rev. B, 64 094409 (2001)). Lanthanum manganites with divalent substitutions have attracted considerable interest in recent years due to double exchange mediated ferromagnetism, metallic conductivity, and giant magnetoresistance (Ramirez, A. P., J. Phys.: Condens. Mater 9, 8171 (1997)). The manganites are potential candidates for ME composites because of (i) high magnetostriction and (ii) metallic conductivity that eliminates the need for a foreign electrode at the p-m interface.
The art of magnetoelectric composites is well developed as shown by U.S. Pat. Nos. 4,769,599 and 5,130,654 to Mermelstein, U.S. Pat. Nos. 5,512,196 and 5,856,770 to Mantese et al, U.S. Pat. No. 5,675,252 to Podney and U.S. Pat. No. 6,279,406 to Li et al. Additional art is shown in U.S. publications 2001/0040450 A1 and 2001/0028245 A1 to Li et al. What is apparent from this art is that while the concepts are well developed, the composites need an improvement in the magnetoelectric properties. All of these patents and applications are incorporated by reference herein.